1. Field of the Invention
The present invention relates to a liquid crystal display unit, and more particularly to an in-plane-switching (IPS) active-matrix liquid crystal display unit.
2. Description of the Related Art
Liquid crystal display (LCD) units are generally characterized by low-profile shapes, lightweight structures, and low-power requirements. Particularly, active-matrix liquid crystal display (AM-LCD) units which comprise a two-dimensional matrix of pixels energizable by active devices are highly promising as high-image-quality flat panel displays. Among those active-matrix liquid crystal display units which are finding widespread use are thin-film-transistor liquid crystal display (TFT-LCD) units which employ thin-film transistors (TFTs) used as active devices for switching individual pixels.
Conventional AM-LCD units utilize a twisted-nematic (TN) electrooptical effect, and comprise a liquid crystal layer sandwiched between two substrates. The liquid crystal layer is activated when an electric field is applied substantially perpendicularly to the substrates.
U.S. Pat. No. 3,807,831 discloses an in-plane-switching liquid crystal display unit having a liquid crystal layer which is activated when an electric field is applied substantially parallel to two substrates which sandwich the liquid crystal layer therebetween, the liquid crystal display unit including interleaved arrays of alternate parallel electrodes.
Japanese patent publication No. 21907/88 reveals an AM-LCD unit based on a TN electrooptical effect and including interleaved or interdigitating arrays of alternate parallel electrodes for the purpose of reducing parasitic capacitance between a common electrode and a drain bus line or between a common electrode and a gate bus line.
FIG. 1 of the accompanying drawings shows a conventional in-plane-switching liquid crystal display unit. The illustrated conventional liquid crystal display unit comprises a liquid crystal layer sandwiched between two glass substrates 11, 12, and interdigitating arrays of alternate parallel electrodes 70 mounted on one of the glass substrates 11. When a voltage is applied between the electrodes 70, a liquid crystal activating electric field E1 is generated parallel to the glass substrates 11, 12 and perpendicularly to the interdigitating teeth of the electrodes 70 for thereby changing the orientation of liquid crystal molecules 21. Therefore, the application of the voltage between the electrodes 70 is effective to control the transmittance of light through the liquid crystal layer. The term xe2x80x9cthe orientation of liquid crystal moleculesxe2x80x9d used in this specification means the direction of the longer axis of liquid crystal molecules.
With the in-plane-switching liquid crystal display unit shown in FIG. 1, it is necessary that when the voltage is applied, the liquid crystal molecules be rotated in a certain direction in order to achieve stable displays. To meet such a requirement, it is customary to initially orient the liquid crystal molecules in a direction that is slightly shifted from a direction perpendicular to the liquid crystal activating electric field. Specifically, the liquid crystal molecules are initially oriented at an angle of xcfx86LC0 ( less than 90xc2x0) with respect to a direction perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. In the specification, the direction of the electric field and the orientation of the liquid crystal molecules will be described in the range of from xe2x88x9290xc2x0 to 90xc2x0 (the counterclockwise direction being positive) with respect to a reference direction (xcfx86=0) which is perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. As described later on, in order to accomplish sufficient display contrast, it is necessary to rotate the liquid crystal molecules 45xc2x0 from the initial orientation. Therefore, it is preferable to orient the liquid crystal molecules at an angle of xcfx86LC0 in the range of 45xc2x0xe2x89xa6xcfx86LC0 less than 90xc2x0. In the in-plane-switching liquid crystal display unit shown in FIG. 1, the initial orientation of the liquid crystal molecules is slightly shifted clockwise (as viewed from the upper substrate 12) from the parallel pairs of the interdigitating teeth of the electrodes. When the voltage is applied, therefore, the liquid crystal molecules are rotated clockwise as indicated by the arrows.
The transmittance T of light passing through the liquid crystal cell shown in FIG. 1 which is sandwiched between two confronting polarizers whose axes of polarization transmission (directions of polarization) are perpendicular to each other is expressed by the following equation (1):                     T        =                              1            2                    ⁢                      sin            2                    ⁢                      {                          2              ⁢                              (                                                      φ                    P                                    -                                      φ                    LC                                                  )                                      }                    ⁢                                    sin              2                        ⁡                          (                                                π                  ⁢                                      xe2x80x83                                    ⁢                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  nd                                λ                            )                                                          (        1        )            
where xcfx86LC represents the orientation of the liquid crystal molecules when a voltage is applied thereto, xcfx86P the direction of the axis of transmission of the polarizer on which the light falls, xcex94n the refractive index anisotropy of the liquid crystal layer, d the thickness of the cell (the thickness of the liquid crystal layer, and xcex the wavelength of the light. The direction xcfx86A of the axis of transmission of the polarizer from which the light exits is expressed by xcfx86A=xcfx86P+90xc2x0 or xcfx86A=xcfx86Pxe2x88x9290xc2x0. It is possible to control the transmittance of the light by varying the orientation xcfx86LC of the liquid crystal molecules with a liquid crystal activating electric field parallel to the substrates based on the above equation (1). If the direction of the axis of transmission of one of the polarizers and the initial orientation of the liquid crystal molecules are in agreement with each other (xcfx86LC0=xcfx86P or xcfx86LC0=xcfx86A), then the liquid crystal display unit is brought into a dark display state when no voltage is applied. If the orientation of the liquid crystal molecules is rotated substantially 45xc2x0 under a liquid crystal activating electric field, then the transmittance becomes highest, and the liquid crystal display unit is brought into a bright display state. Of course, the polarizers may be so arranged that the liquid crystal display unit will be brought into a dark display state when a voltage is applied.
It has been assumed for the sake of brevity that the liquid crystal molecules in the liquid crystal layer between the upper and lower substrates are uniformly rotated. Discussions based on such a simplified model do not essentially affect the principles of the present invention. Actually, however, those liquid crystal molecules which are held in contact with the surfaces of the upper and lower substrates are relatively firmly fixed in position, and do not basically change their orientation, whereas those liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates change their orientation to a greater extent. In view of these practical considerations, the in-plane angle xcfx86LC through which the liquid crystal molecules rotate under an applied electric field is represented as a function of coordinates in the transverse direction of the liquid crystal layer.
In order to accomplish sufficient display contrast, the orientation of the liquid crystal molecules may be rotated substantially 45xc2x0 in the entire liquid crystal layer. However, for the reasons described above, the liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates are actually rotated more than 45xc2x0.
Published Japanese translation No. 505247/93 of a PCT international publication (International publication No. WO91/10936) describes improvements of angle of view characteristics, which have been poor in TN liquid crystal display devices, achieved by the in-plane-switching liquid crystal display unit. Because of their excellent angle of view characteristics, in-plane-switching active-matrix liquid crystal display units have recently been considered as a candidate for large-size display monitors.
FIG. 2 of the accompanying drawings shows the transmittance of the liquid crystal display unit shown in FIG. 1 as it varies when the applied voltage is changed, with respect to various observational directions in which the liquid crystal display unit is observed. The observational directions are defined as xcfx86obs and xcex8obs where xcfx86obs is an angle of orientation with respect to a direction perpendicular to the direction of the electrodes and xcex8obs is an angle of tilt from a direction perpendicular to the substrates. A sample liquid crystal cell used in obtaining the measurements shown in FIG. 2 was arranged such that xcfx86LC=85xc2x0, xcfx86P=85xc2x0, and xcfx86A=xe2x88x925xc2x0. The sample liquid crystal cell had interdigitating arrays of alternate parallel electrodes, including interdigitating teeth each having a width of 5 xcexcm with adjacent ones of the interdigitating teeth being spaced 15 xcexcm from each other. The sample liquid crystal cell had a liquid crystal material whose refractive index anisotropy xcex94n is 0.067. The sample liquid crystal cell had a thickness of 4.9 xcexcm. It can be seen from FIG. 2 that the transmittance does not change largely depending on the observational direction. Therefore, the in-plane-switching liquid crystal display unit shown in FIG. 1 has excellent angle of view characteristics.
However, the in-plane-switching liquid crystal display unit shown in FIG. 1 suffers a problem in that displayed images may look bluish or reddish to a viewer depending on the observational direction.
FIG. 3 of the accompanying drawings shows the transmittance of the liquid crystal display unit shown in FIG. 1 as it varies with the wavelength with respect to various observational directions when the liquid crystal display unit is brought into a bright display state. The measurements shown in FIG. 3 were obtained from the same liquid crystal cell as the one used to obtain the measurements shown in FIG. 2. In the liquid crystal cell, the orientation xcfx86LC of the liquid crystal molecules is 40xc2x0 because when the liquid crystal cell is brought into a bright display state, i.e., when a voltage is applied, the orientation xcfx86LC changes about 45xc2x0 from the initial orientation xcfx86LC0=85xc2x0. It can be understood from FIG. 3 that when the liquid crystal cell is brought into a bright display state, the peak of the transmission spectrum at the observational direction xcfx86obs=40xc2x0 is shifted toward shorter wavelengths, making displayed images bluish, and the peak of the transmission spectrum at the observational direction xcfx86obs=xe2x88x9250xc2x0 is shifted toward longer wavelengths, making displayed images reddish. The same tendency was observed at those observational directions which are 180xc2x0 spaced from the above observational directions.
As described above, while the in-plane-switching liquid crystal display unit has much better characteristics than the conventional TN liquid crystal display units with regard to display contrast and freedom from gradation reversal, it suffers the problem of tilts depending on the observational direction.
In the above liquid crystal cell, the liquid crystal molecules are directed at the initial orientation xcfx86LC0=85xc2x0 in the absence of any applied voltage. When a voltage is applied to bring the liquid crystal cell into a bright display state, the orientation xcfx86LC of the liquid crystal molecules is 40xc2x0 because the orientation xcfx86LC changes about 45xc2x0 from the initial orientation xcfx86LC0=85xc2x0. The direction in which displayed images look bluish to the viewer corresponds to this orientation xcfx86LC of the liquid crystal molecules, and the direction in which displayed images look reddish to the viewer corresponds to the orientation perpendicular to the orientation xcfx86LC. In a display mode based on birefringence, as achieved by the above liquid crystal cell, light having a wavelength which satisfies the relationship of xcex94nxc2x7d=xcex/2 passes most efficiently through the liquid crystal cell, as can be seen from the equation (1). The tinting depending on the angle of view, i.e., the angle at which the liquid crystal cell is observed, is caused by the dependency of the birefringence (xcex94nxc2x7d) of the liquid crystal layer on the angle of view.
The dependency of the birefringence of the liquid crystal layer on the angle of view will be described in detail below.
It is assumed that the angle formed between the direction of travel of light and the longitudinal direction of liquid crystal molecules is represented by xcex82, the refractive index with respect to an ordinary ray of light which is vibrated (polarized) in a direction perpendicular to a direction called the optic axis of crystal is represented by no, and the refractive index with respect to an extraordinary ray of light which is vibrated (polarized) parallel to the optic axis is represented by ne. Effective refractive index anisotropy xcex94nxe2x80x2 when light is obliquely applied to the liquid crystal cell is given by the following equation (2):                               Δ          ⁢                      xe2x80x83                    ⁢                      n            xe2x80x2                          =                                                            n                e                            ⁢                              n                o                                                                                                          n                    e                                    ⁢                                      cos                    2                                    ⁢                                      θ                    2                                                  +                                                      n                    o                                    ⁢                                      sin                    2                                    ⁢                                      θ                    2                                                                                -                      n            o                                              (        2        )            
When light is applied perpendicularly to the liquid crystal cell, since xcex82=90xc2x0, the effective refractive index anisotropy xcex94nxe2x80x2 is given as xcex94nxe2x80x2=nexe2x88x92no. In the direction in which displayed images look bluish to the viewer, because the angle of view is tilted to the longitudinal direction of liquid crystal molecules, the angle xcex82 becomes xcex82 less than 90xc2x0 and xcex94nxe2x80x2 becomes smaller. In the direction in which displayed images look reddish to the viewer, because the angle of view is tilted to a direction perpendicular to the longitudinal direction of liquid crystal molecules, the angle xcex82 remains xcex82=90xc2x0 and xcex94nxe2x80x2=xcex94n. FIGS. 4A and 4B illustrate the refractive index anisotropy as it varies with the angle of view.
When light is applied obliquely to the liquid crystal cell, since the substantial thickness dxe2x80x2 of the liquid crystal layer is given by dxe2x80x2=d/cosxcex8obs, the substantial thickness dxe2x80x2 becomes larger independent of the direction in which the angle of view is tilted.
Because of changes of both the refractive index anisotropy and the thickness of the liquid crystal layer, the birefringence (xcex94nxe2x80x2xc2x7dxe2x80x2) varies, changing the tint depending on the angle of view.
Table 1 shown below contains details of the tinting.
As described above, the conventional in-plane-switching liquid crystal display units cannot avoid tinting of displayed images in certain directions.
In view of the experimental data and considerations described above, the inventors have made the present invention in efforts to suppress tinting in in-plane-switching active-matrix liquid crystal display units.
It is therefore an object of the present invention to provide an in-plane-switching liquid crystal display unit which suffers minimum tinting of displayed images due to changes in the angle of view and can display high-quality images.
According to a first aspect of the present invention, there is provided an in-plane-switching liquid crystal display unit comprising a two-dimensional matrix of pixel regions each including two auxiliary regions capable of compensating for tinting characteristics of each other. With this arrangement, the directions in which displayed images look bluish and reddish compensate for each other, thereby suppressing tints of the displayed images due to changes in the angle of view, i.e., the angle at which the liquid crystal display unit is observed.
According to a second aspect of the present invention, there is provided an in-plane-switching liquid crystal display unit comprising a two-dimensional matrix of pixel regions each including a first auxiliary region having liquid crystal molecules directed in a first orientation when no electric field is applied thereto, a second auxiliary region having liquid crystal molecules directed in a second orientation extending at 90xc2x0 with respect to the first orientation when no electric field is applied thereto, and electric field generating means for generating an in-plane electric field in a liquid crystal sealed layer and applying the in-plane electric field to the liquid crystal molecules to rotate the liquid crystal molecules in one direction while maintaining the first orientation and the second orientation at 90xc2x0 with respect to each other. With this arrangement, when the liquid crystal display unit changes from a dark display state to a bright display state, the directions in which displayed images look bluish and reddish compensate for each other, thereby suppressing tints of the displayed images due to changes in the angle of view.
According to a third aspect of the present invention, in the liquid crystal display unit according to the second aspect, the electric field generating means comprises a plurality of parallel pairs of electrodes disposed in the first auxiliary region and the second auxiliary region, the electrodes disposed in the first auxiliary region extending at 90xc2x0 with respect to the electrodes disposed in the second auxiliary region. When a voltage is applied, the liquid crystal molecules are rotated in one direction while their first and second orientations are maintained at 90xc2x0 with respect to each other. Consequently, the directions in which displayed images look bluish and reddish compensate for each other, thereby suppressing tints of the displayed images due to changes in the angle of view.
According to a fourth aspect of the present invention, in the liquid crystal display unit according to the second aspect, the electric field generating means comprises a plurality of parallel pairs of electrodes, the electrodes extending straight in the first auxiliary region and the second auxiliary region, and wherein the liquid crystal molecules are oriented at 45xc2x0 with respect to a direction in which the electrodes extend in the first auxiliary region and the second auxiliary region when no electric field is applied thereto. This arrangement is also effective in suppressing tints of the displayed images due to changes in the angle of view.
According to a fifth aspect of the present invention, the liquid crystal display unit according to the second aspect further comprises a front substrate and a rear substrate, the pixel regions being disposed between the front substrate and the rear substrate, the liquid crystal molecules have a substantially nil pretilt angle with respect to the front substrate and the rear substrate. With this arrangement, the liquid crystal molecules in the first auxiliary region and the second auxiliary region operate stably.
According to a sixth aspect of the present invention, the liquid crystal display unit according to the second aspect further comprises a front substrate and a rear substrate, the pixel regions being disposed between the front substrate and the rear substrate, wherein the liquid crystal molecules have pretilt angles in a spray-type pattern with respect to the front substrate and the rear substrate, and the pretilt angles of liquid crystal molecules near the front substrate and the rear substrate are different from those of other liquid crystal molecules. With this arrangement, the liquid crystal molecules in the first auxiliary region and the second auxiliary region operate stably.
According to a seventh aspect of the present invention, there is provided an in-plane-switching liquid crystal display unit comprising a two-dimensional matrix of pixel regions each including a first auxiliary region having liquid crystal molecules directed in a first orientation when no electric field is applied thereto, a second auxiliary region having liquid crystal molecules directed in a second orientation which is the same as the first orientation when no electric field is applied thereto, and electric field generating means for generating an in-plane electric field in a liquid crystal sealed layer and applying the in-plane electric field to the liquid crystal molecules to rotate the liquid crystal molecules in opposite directions while maintaining the first orientation and the second orientation in symmetric relationship. With this arrangement, when the liquid crystal display unit is in a bright display state, since the liquid crystal molecules in the first and second auxiliary regions are rotated in opposite directions through substantially 45xc2x0 with respect to their initial orientations, the orientations of the liquid crystal molecules in the first and second auxiliary regions lie at 90xc2x0 with respect to each other. Consequently, the directions in which displayed images look bluish and reddish compensate for each other, thereby suppressing tints of the displayed images due to changes in the angle of view. The orientations of the liquid crystal molecules in the first and second auxiliary regions lie at 90xc2x0 with respect to each other only when the liquid crystal display unit is fully in a bright display state. However, even when the liquid crystal display unit displays intermediate gradations, the tinting compensation is partly achieved to reduce tints of the displayed images much better as compared with the conventional liquid crystal display unit. Furthermore, the liquid crystal display unit can be manufactured relatively simply because the initial orientations of the liquid crystal molecules are not required to be different from each other in the first and second auxiliary regions.
According to an eighth aspect of the present invention, in the liquid crystal display unit according to the seventh aspect, the electric field generating means comprises a plurality of parallel pairs of electrodes, the electrodes extending in the first auxiliary region and the second auxiliary region and being bent to a V shape at a boundary between the first auxiliary region and the second auxiliary region. With this arrangement, the boundary where the electrodes are bent to the V shape divides the two auxiliary regions where the liquid crystal molecules are rotated in opposite directions.
According to a ninth aspect of the present invention, in the liquid crystal display unit according to the seventh aspect, the electric field generating means comprises a plurality of pairs of confronting electrodes each having a longer arm and a shorter arm which extend at a predetermined angle with respect to each other and define a rectangular region, the pairs of confronting electrodes being inverted in the first auxiliary region and the second auxiliary region. The pairs of confronting electrodes each having a longer arm and a shorter arm define a rectangular region such as an elongate rectangular region, a parallelogrammatic region, or a trapezoidal region. Therefore, it is possible to generate an electric field slightly tilted with respect to the shorter arms in the regions surrounded by the electrode pairs. Since the direction in which the electric field is tilted is determined by the layout of the electrode pairs, the liquid crystal molecules are rotated in opposite directions in the two auxiliary regions by inverting the layout of the electrode pairs in the auxiliary regions.
According to a tenth aspect of the present invention, in the liquid crystal display unit according to the ninth aspect, the shorter arms of the electrodes are slightly tilted with respect to a direction perpendicular to each of the first and second orientations of the liquid crystal molecules when no electric field is applied. With this arrangement, the direction of rotation of the liquid crystal molecules is stable even in the vicinity of the shorter arms, making the liquid crystal display unit operate stably, and increasing an allowable range of registration errors in a process of manufacturing the liquid crystal display unit.
According to an eleventh aspect of the present invention, in the liquid crystal display unit according to the seventh aspect, the electric field generating means comprises a plurality of parallel pairs of electrodes disposed in the first auxiliary region and the second auxiliary region, the electrodes disposed in the first auxiliary region extending at 90xc2x0 with respect to the electrodes disposed in the second auxiliary region, and wherein each of the first and second orientations extends parallel to a direction bisecting an angle formed between a direction in which the electrodes of the parallel pairs extend in the first auxiliary region and a direction in which the electrodes of the parallel pairs extend in the second auxiliary region. This arrangement is also effective in suppressing tints of the displayed images due to changes in the angle of view.
According to a twelfth aspect of the present invention, in the liquid crystal display unit according to the seventh aspect, wherein each of the first and second orientations is substantially the same as a direction in which a liquid crystal material flows when the liquid crystal material is introduced into the first and second auxiliary regions. With this arrangement, a period of time required to introduce the liquid crystal material can be reduced, and an orientation defect called a flow orientation which would otherwise occur after the liquid crystal material is introduced is minimized.
According to a thirteenth aspect of the present invention, the liquid crystal display unit according to the seventh aspect further comprises a front substrate and a rear substrate, the pixel regions being disposed between the front substrate and the rear substrate, the liquid crystal molecules have a substantially nil pretilt angle with respect to the front substrate and the rear substrate. With this arrangement, the liquid crystal molecules in the first and second auxiliary regions operate stably.
According to a fourteenth aspect of the present invention, the liquid crystal display unit according to the seventh aspect further comprises a front substrate and a rear substrate, the pixel regions being disposed between the front substrate and the rear substrate, wherein the liquid crystal molecules have pretilt angles in a spray-type pattern with respect to the front substrate and the rear substrate, and the pretilt angles of liquid crystal molecules near the front substrate and the rear substrate are different from those of other liquid crystal molecules. This arrangement is also effective to operate the liquid crystal molecules stably in the first and second auxiliary regions.